This invention relates to a method for precision turning (smoothing, finish turning) horizontally clamped workpieces, which are inclined to sag, by means of a numerically controlled lathe. Further, the invention relates to an apparatus for carrying out this method.
With the known lathes of this kind, sagging of the horizontally clamped workpiece is prevented by self-centering back rests which consist of two arms bearing rollers and of which the closing and opening movements describe an arc. A third roller is mounted between the two arms on a displaceable center sleeve and is as a rule located vertically below the workpiece. The closing and opening process is initiated by the numerical control of the lathe. The initial setting force is produced by means of a hydraulic pump and is variable. These self-centering back rests are mounted on an auxiliary bed of the lathe and can, after release of their clamping on the auxiliary bed guide tracks, be displaced into any position parallel to the turning centre of the lathe and reclamped.
According to the requirements of the workpiece to be worked, several stationary rests can be displaced over the workpiece length. Also known is the practice of letting a back rest travel with the tool so that the workpiece is always supported at the point at which the cutting tool is located.
The arrangement of such back rests is of course relatively elaborate especially as their position and their opening and closing rhythm must be programmed. Moreover, the lathe must have an auxiliary bed with auxiliary guide tracks for the back rests.